Update "Getting ready" advice for OpenRefine

[ostephens]

Suggested changes:

• Emphasise the use of the OpenRefine versions that include Java - these are now the recommended install options by the OpenRefine project and should run more smoothly than the ones without embedded java
• Remove mention of the MyBinder version as this is now quite outdated and has been reported as ineffective

Additional suggestions or counter proposals welcome

[ostephens]

Note that OpenRefine project currently working on Windows installer that will work in a more intuitive way for users - once that's available these instructions will need further updating